The present invention relates generally to the field of measuring the size and velocity of particles suspended in a fluid medium and more particularly to laser Doppler velocimeters used for transmitting light to and from the measurement zone.
In U.S. patent application Ser. No. 07/432,710 entitled, "Method and Apparatus For Measuring Small Particle Size Distribution" filed by Michael N. Trainer, William Leslie Wilcock and Brian M. Ence and assigned to General Signal Corporation, the assignee of this application, a method and apparatus is described for measuring the size distribution of very small particles. The apparatus described includes a dynamic scattering instrument having an optical coupler. The integrated optical waveguide Doppler velocimeter of the present invention is especially useful in such an apparatus and method. As described in U.S. application Ser. No. 07/432,710, fiber optic Doppler anemometers of the type disclosed in U.S. Pat. No. 4,637,716 to Auweter et al and U.S. Pat. No. 4,818,071 to Dyott are also quite useful in such an apparatus and method. However, applicants have discovered that it would be desirable to have an optical device in such an environment which is more rugged than fiber optic couplers, which reduce environmentally induced optical phase noise, provides better polarization stability than a fiber optic coupler, and provides an increased signal to noise ratio when compared with a fiber optic coupler. Applicants further discovered that it would be desirable to provide an optical device which could be easily integrated with other optical devices such as frequency shifters and modulators. Heretofore there has been no known device for providing such advantages.